1. Field of the Invention
The present invention relates to an image forming apparatus to which an electrophotographic system is applied, and a method of manufacturing an electronic circuit using the same.
2. Description of the Related Art
Conventionally, etching and screen printing methods have been heavily used in forming an electronic circuit applied to a wiring board, semiconductor element and the like. When the etching method is used to form a conductor pattern, exposure masks are necessary for respective layers. In the screen printing method, exclusive masks are necessary as well. Since tremendous amounts of money and time are required in designing and manufacturing these masks, manufacturing cost and manufacturing time of the electronic circuit are affected significantly. In order to solve such problems of the conventional methods, the method of forming the conductor pattern and the like by using an electrophotographic system has been developed (refer to Japanese Patent Laid-open Application No. Hei 7-263841, Japanese Patent Laid-open Application No. 2001-284769, for example).
The conductor pattern to which the electrophotographic system is applied is formed as follows, for example. First, toner (charged particles) which contains metal fine particles in insulating resin particles made of thermosetting resin such as epoxy resin is used to selectively form a plating base layer on a substrate. This plating base layer is subjected to electroless plating, whereby a metal conductor layer having a desired circuit pattern is formed (refer to Japanese Patent Application Laid-open No. 2004-048030, for example). The electrophotographic system is also applied to the formation of an insulating layer. The insulating layer is formed by bringing the toner made of the insulating resin particles into adhesion onto the substrate, and thermally fixing this insulating resin particle layer.
An image forming apparatus used in manufacturing the electronic circuit is mainly constituted of an exposure unit forming an electrostatic latent image on a photoconductive drum, a developing unit bringing the toner into adhesion with the electrostatic latent image to form a visible image, a transferring unit transferring the visible image by the toner (toner image) onto the substrate, and a fixing unit fixing the toner image transferred onto the substrate. Although several methods, such as an electrostatic transfer method, pressure transfer method, adhesive transfer method and the like, are known as the method of transferring the toner image formed on the surface of the photoconductive drum onto the substrate, the electrostatic transfer method which is suitable for high speed and whose structure is relatively simple is heavily used in general copying machines, laser printers and the like.
The electrostatic transfer method is the method of transferring the toner image formed on the surface of a photoconductor onto the substrate by electrostatic force. When negative-charged toner is transferred from the surface of the photoconductor onto the substrate, for example, a voltage is applied between the photoconductor and the substrate so that positive charges whose polarity is reverse to that of the toner are added to the back face side of the substrate. By generating an electric field between the photoconductor and the substrate based on the application of the voltage, the negative-charged toner is moved from the surface of the photoconductor onto the substrate by the electrostatic force. Thus, the toner image formed on the surface of the photoconductor is electrostatically transferred onto the substrate.
When the electronic circuit is manufactured using the electrophotographic system, it is essential that the insulating layer is formed on the metal conductor layer using the insulating resin particles as the toner. When the electrostatic transfer method is applied to formation process of the insulating layer using the insulating resin particles, the problem is caused that the transfer of the insulating resin particles (toner) onto the metal conductor layer is performed less sufficiently toward the rear side in the moving direction of the substrate. Namely, although the insulating resin particles are transferred comparatively well on the front side in the moving direction of the metal conductor layer, the insulating resin particles cease to be transferred from the middle of the metal conductor layer and only the defective insulating layer is obtainable. This is supposed to be caused by an electrostatic induction phenomenon of the conductor by the electric field.
Further, when the electrophotographic system is applied to the formation of the insulating layer, it is necessary to form the insulating layer to have a desired pattern with high accuracy on the conductor layer having various circuit patterns. However, since the conventional image forming apparatus has low accuracy of positioning of respective patterns, it has the problem that accuracy of form and the like of the electronic circuit tend to lower. The electrophotographic system is also applied to the manufacturing of a wiring board having multilayer wiring structure. The wiring board having the multilayer wiring structure is manufactured by layering the conductor layers and insulating layers for plural times. In such a case, the positioning of the respective patterns becomes further difficult.
In formation process of the conductor layer to which the electrophotographic system is applied, the plating base layer is formed using the toner containing the metal particles in the insulating resin particles, as described above. The toner containing the metal particles has low electrical resistance, and is difficult to control the amount of charges as compared to normal toner for electrophotography. Additionally, the thermosetting resin such as epoxy resin which constitutes the metal particle-containing toner has more functional groups as compared to thermoplastic resin such as styrene resin and polyester resin which are used for the normal toner for electrophotography. It is therefore difficult to maintain the amount of charges especially in a high-humidity environment because of moisture absorption.
Under the condition where the electrical resistance of the toner is low and the amount of charges is decreased, the phenomenon in which the toner adheres to the parts without the electrostatic latent image on the photoconductor, that is, the so-called “fog” tends to be caused. When the “fog” is caused, the toner adheres to the parts other than the developed circuit pattern, whereby the problems such as a short circuit are caused in the circuit by the conductor layer. This results in lower reliability and a lower manufacturing yield of the electronic circuit which is applied to the wiring board and the like.
As described thus far, the manufacturing process of the electronic circuit using the conventional image forming apparatus has the problem that defective shapes of the respective layers constituting the electronic circuit, such as transfer failure of the insulating layer and the like, positioning failure of the respective layers, the fog in forming the conductor layer (plating base layer) and the like, tend to be caused. The transfer failure due to the electrostatic induction phenomenon of the conductor is not limited to the insulating layer. The transfer failure may be caused similarly when the electrophotographic system is applied to the formation of a semiconductor layer, for example, depending on the magnitude of the electric field to be applied. Accordingly, the technique of forming the respective layers constituting the electronic circuit to have desired patterns favorably and securely is required.